The content of silica dissolved in the aluminate liquor must be controlled to prevent scaling of pipes in the Bayer circuit and to control the purity of the trihydrate produced. The vigilance necessary for this control varies depending on the nature of the treated bauxite. Paradoxically, this control is most difficult for bauxites with a low content of reactive silica.
A first type of desilication is carried out on the entire liquor itself. For example, it is carried out immediately after digestion with a sufficiently long residence time for the quantity of dissolved silica to reach a critical supersaturation threshold, which triggers the desilication reaction by precipitation of siliceous compounds and leads to a drop in the concentration of silica in the liquor to an acceptable level, just before separation of the insoluble residues. It can also be carried out before digestion, by locally introducing precipitation germs: for example, in the process described by U.S. Pat. No. 6,086,834 (ALCAN), in which the liquor must pass through a bed composed of a mix of sand and sodalite, or in the process described in French application No. 03 11909 (ALUMINIUM PECHINEY) in which a calcium based compound (typically comprising hydrogarnets) is introduced into the aluminate liquor upstream of the heat exchangers.
A second type of desilication is called “predesilication” because the ground bauxite is brought into contact with an alkaline solution before digestion. U.S. Pat. No. 3,481,705, AU Patent No. 0 474 596 and U.S. Pat. No. 4,426,363 describe a predesilication treatment, where the alkaline solution may be a sodium hydroxide solution, optionally mixed with sodium carbonate, a potassium hydroxide solution or, preferably, a Bayer aluminate liquor. Using the latter is preferred because it is not expensive: a small part of the Bayer spent liquor is drawn off and conducted to the wet grinding of the bauxite. The thick slurry resulting from wet grinding is maintained in a tank typically a few hours before digestion. The silica contained in the bauxite is dissolved in this liquor aliquot, which quickly reaches a supersaturation threshold from which the precipitation of desilication products (DSP) occurs. The DSP are then eliminated with the insoluble residues of bauxite digestion. Predesilication also acts directly on bauxite components added into the Bayer circuit, but unlike the first type of treatments, it cannot be used efficiently to reduce the silica content of the liquor quickly at a precise location of the Bayer circuit. For this reason, both types of desilication treatments are generally combined on the same Bayer process.
This invention relates to an improvement to the predesilication treatment. The predesilication treatment may be considered as being a preventive treatment carried out on the bauxite that is introduced into the Bayer circuit. However, this type of treatment can only be efficient if it prevents the liquor from reaching a given critical concentration threshold of dissolved silica before crystallization—typically a silica/caustic soda ratio of about 0.7% for factories with a caustic soda concentration of about 155 g/l (note in the following that such ratios are referenced to the weight of caustic soda “% ctq”, or in fact “% SiO2 ctq”)—so that in particular the purity of the trihydrate produced can be controlled.
It is found that predesilication treatments recommended by different documents according to prior art are often apparently efficient, but cannot always achieve a sufficiently low content of dissolved silica in the liquor.
A perfect predesilication would consist of transforming all soluble siliceous phases of bauxite (for example kaolinite) into the most stable possible phase (DSP) under digestion conditions. In this way, the concentration of silica in the liquor at the output from digestion would be less than or equivalent to the DSP solubility concentration (solubility between 0.5 and 0.9% ctq depending on digestion conditions). However, in the article “Effect of Predesilication and Digestion Conditions on Silica Level in Bayer Liquor”—Light Metals 2004 TMS, 2004, the authors (E. TIZON, Ph. CLERIN, B. CRISTOL) demonstrated that the supposedly insoluble phases (DSP) created during predesilication are not all stable under digestion conditions, particularly for residence times typically used in the industry (6 to 10 h), and that their dissolution during the digestion may finally participate in the increase in the content of silica dissolved in the liquor. Thus, under some conditions, in predesilication as it is carried out in the prior art, the silica content dissolved in the aluminate liquor downstream of digestion cannot be efficiently controlled.
It has also been demonstrated that solubility of DSP increases greatly when the concentration of alumina increases. Thus, this loss of efficiency of predesilication may in particular lead to production losses during digestion (the alumina concentration is limited by the silica concentration).